Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Optical metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition and other parameters of nanoscale structures.
As devices (e.g., logic and memory devices) move toward smaller nanometer-scale dimensions, characterization becomes more difficult. Devices incorporating complex three-dimensional geometry and materials with diverse physical properties contribute to characterization difficulty. In addition to accurate device characterization, measurement consistency across a range of measurement applications and a fleet of metrology systems tasked with the same measurement objective is also important. If measurement consistency degrades in a manufacturing environment, consistency among processed semiconductor wafers is lost and yield drops to unacceptable levels. Matching measurement results across applications and across multiple systems (i.e., tool-to-tool matching) ensures that measurement results on the same wafer for the same application yield the same result.
A typical calibration approach for model based measurement systems consists of measuring a number of film/substrate systems of known thickness and dielectric function. A regression is performed on machine parameters until the combination of parameters returns the expected values for thickness and/or dielectric function. In one example, a set of film wafers having a silicon dioxide layer on crystalline silicon over a range of thicknesses is measured and a regression is performed on the machine parameters until the machine returns the best match for thickness and/or refraction index for the given set of films. Other examples are described in U.S. Pat. Pub. No. 2004/0073398 entitled, “Methods and Systems for Determining a Critical Dimension and a Thin Film Characteristic of a Specimen,” which is incorporated by reference as if fully set forth herein. This calibration procedure may be applied across a fleet of measurement systems using the same set of wafers. These wafers are sometimes referred to as transfer standards.
Machine parameters are often calibrated based on thin film measurements because thin film systems (e.g., silicon dioxide on crystalline silicon) can be manufactured with well-known optical constants, clean interfaces, and low surface roughness that enable measurement of wafer characteristics with a degree of repeatability near the sensitivity of the measurement systems being calibrated. However, the accuracy of a metrology system calibrated based on reference wafers is typically limited to wafers with properties that closely match those of the reference wafer. Thus, the effectiveness of calibration based on thin film measurements may be limited in different measurement applications.
To achieve a high level of measurement consistency across a fleet of metrology systems with a reference wafer (or set of reference wafers), calibration experiments involving the reference wafer must be performed in a carefully controlled environment that matches the environmental conditions in place when the reference wafer was originally characterized. This may be difficult to achieve in a manufacturing environment and lead to loss of consistency among metrology systems. In addition, an expensive reference wafer set must be maintained in the manufacturing environment. Risks of wafer breakage or degradation potentially jeopardize the integrity of the calibration process.
Tool-to-tool matching and maintaining tool measurement consistency over time, over maintenance cycles, and over a wide range of measurement applications are core challenges in the development of an optical metrology system that meets customer requirements of the semiconductor industry. Process and yield control in both the research and development and manufacturing environments demands tool-to-tool consistency of measurement results on the order of the measurement repeatability. Thus, methods and systems for improved tool-to-tool matching and consistent measurement performance over a wide range of measurement applications are desired.